Studies were started on the mechanism of the transfer of cholesterol between plasma lipoproteins and erythrocytes. As a first step, a method for the replacement of almost all of the membrane cholesterol by labelled cholesterol was developed, and the equivalence of the labelled and native cholesterol was demonstrated. Also, a method was developed for the removal of up to 77% of the membrane cholesterol, which should be of value in determining the function of this material in the membrane. Fluorescence measurements showed that the mean distance between the membrane protein and lucensomysin, a molecule which would be expected to accumulate in the cholesterol-containing regions of the membrane, is 27 angstroms.